Industrial process industries primarily rely upon energy sources that include one or more combustion processes. Such combustion processes include operation of a furnace or boiler to generate energy from combustion, which is then used for the process. While combustion provides relatively low-cost energy, its use is typically regulated and combustion efficiency is sought to be maximized. Accordingly, one goal of the process management industry is to reduce the production of greenhouse gases by maximizing combustion efficiency of existing furnaces and boilers.
In situ or in-process analyzers are commonly used for the monitoring, optimization, and control of combustion processes. Typically, these analyzers employ sensors that are heated to relatively high temperatures and are operated directly above, or near, the furnace or boiler combustion zone. Known analyzers, such as that sold under the trade designation X-Stream O2 Combustion Flue Gas Transmitter available from Rosemount Analytical Inc. of Solon, Ohio (an Emerson Process Management company), often employ zirconia oxide sensors heated to a temperature above approximately 700° Celsius (1300° Fahrenheit). If the combustion process should suffer a flame out condition, raw fuel and air could be exposed to this sensor which, by virtue of its elevated temperature, could become an ignition source with the possibility of precipitating an explosion.
Known analyzers generally employ a sintered metal or other diffuser positioned between a measurement cell and the process combustion gas to allow the process gas to diffuse to the measurement zone while minimizing flow effects and reducing measurement cell contamination. The diffuser readily allows the process gas to contact the heated measurement cell itself and, in the case of the process combustion gas is replaced by a flammable gas, enables an explosion. This situation can occur if the combustion flame is extinguished and fuel continues to flow.
Some process analyzers are approved for hazardous area operation. Some approvals include those provided by the Canadian Standards Association (CSA), Factory Mutual (FM), ATmosphares EXplosibles (ATEX), et cetera. Typically, hazardous area-approved analyzers include a flame arrestor that is added over the diffuser with the intent of quenching, or otherwise inhibiting, an explosion that might occur in front of the heated measurement cell, thereby preventing the ignition of the larger fuel volume in the boiler or combustion zone. These flame arrestors have been tested and approved in the past. However, it is believed that the safety provided by such arrestors can be improved. Moreover, the utilization of the flame arrestors may inhibit, to some degree, access to the measurement cell thereby increasing measurement lag.
State of the art process safety systems generally provide a flame scanner to alert an operator and/or send an electrical signal indicating that the flame is extinguished and that raw fuel may be flowing. Fully automated systems immediately shut down fuel flow, while manual systems generally require operator intervention.
A potentially hazardous situation can also arise during the initial lighting of the process burner or boiler, where fuel is introduced and an ignition source is used to initiate a flame. In some situations, raw fuel may reach the oxygen sensor (heated by its own heater to a temperature of 700° Celsius) which may provide a source of ignition prior to the intended ignition source. This can cause a potential flash or explosion. Typically, either a flame arrestor is used on the oxygen sensor or the analyzer is not powered during boiler or furnace startup. The non-powered analyzer is completely safe since the oxygen sensor is not heated and thus cannot form an unintended source of ignition. However, since the analyzer is non-functional for 30-45 minutes after startup, the analyzer is unavailable during the critical combustion startup phase. This can waste fuel and allow excessive emissions and inefficiencies. Thus, it is desired to have an analyzer system that provides both safe startup and fault condition operation while remaining readily available.